Transmission and Reception of an Image Using LTE Toolbox and a Single USRP® E310

This example shows how to use the USRP® Embedded Series Radio Support Package with MATLAB® and LTE Toolbox™ to generate a multi-antenna LTE transmission for simultaneous transmit and receive on a single SDR platform. An image file is encoded and packed into a radio frame for transmission, and subsequently decoded on reception. The diagram below shows the setup used:

Refer to the Guided Host-Radio Hardware Setup documentation for details on configuring your host computer to work with the Support Package for USRP® Embedded Series Radio.

Introduction

You can use the LTE Toolbox to generate standard-compliant baseband IQ downlink and uplink reference measurement channel (RMC) waveforms and downlink test model (E-TM) waveforms. These baseband waveforms can be modulated for RF transmission using SDR hardware such as USRP® Embedded Series Radio.

This example imports an image file and packs it into multiple radio frames of a baseband RMC waveform that it generates using the LTE Toolbox. The example creates a continuous RF LTE waveform by using the Repeated Waveform Transmitter functionality with the USRP® radio hardware, whereby the baseband RMC waveform is transferred to the hardware memory on the radio, and transmitted continuously over the air without gaps. As the E310 device is capable of two channel transmission and reception, the example generates and transmits a multi-antenna LTE waveform using LTE Transmit Diversity. The script then captures the resultant waveform using the same E310 hardware platform.

Example Setup

Make sure that LTE Toolbox in installed. You must have an LTE Toolbox license to run this example. If you do not have the LTE Toolbox, install it now to continue with this example.

When you run this example, the first thing the script does is check for the LTE Toolbox.

% Check that LTE Toolbox is installed, and that there is a valid licenseif isempty(ver('lte')) % Check for LST install
error('usrpe3xxLTEMIMOTransmitReceive:NoLST', ...'Please install LTE Toolbox to run this example.');
elseif ~license('test', 'LTE_Toolbox') % Check that a valid license is present
error('usrpe3xxLTEMIMOTransmitReceive:NoLST', ...'A valid license for LTE Toolbox is required to run this example.');
end

The script then configures all of the scopes and figures that will be displayed throughout the example.

The script will then connect to the SDR device to verify the host/hardware connection, and to get information on the specific RF card version that is connected. An information message will be displayed in the command window while the connection to the hardware is established.

% Connect to the SDR device, and get device info
devInfo = radio.info;

## Establishing connection to hardware. This process can take several seconds.

Run Example

You can run this example by executing usrpe3xxLTEMIMOTransmitReceiveML.m. The following sections explain the design and architecture of this example, and what you can expect to see as the code is executed.

Transmitter Design: System Architecture

The general structure of the LTE transmitter can be described as follows:

Import an image file and convert it to a binary stream.

Generate a baseband LTE signal using LTE Toolbox, packing the binary data stream into the transport blocks of the downlink shared channel DL-SCH.

Prepare the baseband signal for transmission using the SDR hardware.

Send the baseband data to the SDR hardware for upsampling and continuous transmission at the desired center frequency.

In order to visualize the benefit of using multi-channel transmission and reception over single-channel, you can reduce the transmitter gain parameter to impair the quality of the received waveform, as shown here:

% TX gain parameter:% Change this parameter to reduce transmission quality, and impair the% signal. Suggested values:% * set to -10 for default gain (-10dB)% * set to -20 for reduced gain (-20dB)%% NOTE: These are suggested values -- depending on your antenna% configuration, you may have to tweak these values.
txsim.Gain = -10;

Prepare Image File

The example reads data from the image file, scales it for transmission, and converts it to a binary data stream.

The size of the transmitted image directly impacts the number of LTE radio frames which are required for the transmission of the image data. A scaling factor of scale = 0.5, as shown below, requires the transmission of 5 LTE radio frames. Increasing the scaling factor will result in the transmission of more frames; conversely, reducing the scaling factor will reduce the number of frames.

The example uses the default configuration parameters defined in TS36.101 Annex A.3 [ 1 ] to generate an RMC by lteRMCDLTool. The parameters within the configuration structure rmc can then be customized as required. The example generates a baseband waveform, eNodeBOutput, a fully populated resource grid, txGrid, and the full configuration of the RMC using lteRMCDLTool. The example uses the binary data stream that was created from the input image file trData as input to the transport coding, and packs it into multiple transport blocks in the Physical Downlink Shared Channel (PDSCH). The number of frames that are generated for transmission is dependent on the image scaling that you set when importing the image file. The generation of the baseband LTE signal is shown in the following code:

The sdrTransmitter uses the transmitRepeat functionality to continuously transmit the baseband LTE waveform in a loop from the DDR memory on the Zynq-Based Radio platform. The applied channel map for the sdrTransmitter is displayed in the command window.

The transmitRepeat function transfers the baseband LTE transmission to the SDR platform, and stores the signal samples in hardware memory. The example then transmits the waveform continuously over the air without gaps until the release method for the transmit object is released. Messages are displayed in the command window to confirm that transmission has started successfully.

sdrTransmitter.transmitRepeat(eNodeBOutput);

## Establishing connection to hardware. This process can take several seconds.

Receiver Design: System Architecture

The general structure of the LTE receiver can be described as follows:

Capture a suitable number of frames of the transmitted LTE signal using SDR hardware.

Determine and correct the frequency offset of the received signal.

Synchronize the captured signal to the start of an LTE frame.

OFDM demodulate the received signal to get an LTE resource grid.

Perform a channel estimation for the received signal.

Decode the PDSCH and DL-SCH to obtain the transmitted data from the transport blocks of each radio frame.

Recombine received transport block data to form the received image.

This example plots the power spectral density of the captured waveform, and shows visualizations of the estimated channel, equalized PDSCH symbols, and received image.

Receiver Setup

The receiver is controlled using the parameters defined in the rxsim structure. The sample rate of the receiver is 15.36MHz, which is the standard sample rate for capturing an LTE bandwidth of 50 resource blocks (RBs). 50 RBs is equivalent to a signal bandwidth of 10 MHz.

The example simplifies the LTE signal reception by assuming that the transmitted PDSCH parameters are known. FDD duplexing mode and a normal cyclic prefix length are also assumed, as well as four cell-specific reference ports (CellRefP) for the MIB decode. The number of actual CellRefP is provided by the MIB. A detailed example of how to perform a blind LTE cell search and recover basic system information from an LTE waveform is given in Cell Search, MIB and SIB1 Recovery (LTE Toolbox).

Signal Capture and Processing

The example uses a while loop to capture and decode bursts of LTE frames. As the LTE waveform is continually transmitted over the air in a loop, the first frame that is captured by the receiver is not guaranteed to be the first frame that was transmitted. This means that the frames may be decoded out of sequence. To enable the received frames to be recombined in the correct order, their frame numbers must be determined. The Master Information Block (MIB) contains information on the current system frame number, and therefore must be decoded. After the frame number has been determined, the PDSCH and DL-SCH are decoded, and the equalized PDSCH symbols are shown. No data is transmitted in subframe 5; therefore the captured data for subframe is ignored for the decoding. The Power Spectral Density (PSD) of the captured waveform is plotted to show the received LTE transmission.

When the LTE frames have been successfully decoded, the detected frame number is displayed in the command window on a frame-by-frame basis, and the equalized PDSCH symbol constellation is shown for each subframe. An estimate of the channel magnitude frequency response between cell reference point 0 and the receive antennae is also shown for each frame.

Things to Try

By default, the example will use multiple antennas for transmission and reception of the LTE waveform. You can modify the transmitter and receiver to use a single antenna and decrease the transmitter gain, to observe the difference in the EVM and BER after signal reception and processing. You should also be able to see any errors in the displayed, received image.

Troubleshooting the Example

General tips for troubleshooting SDR hardware and the Communications Toolbox Support Package for USRP® Embedded Series Radio can be found in Common Problems and Fixes.

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